1. Field of the Invention
This invention relates generally to apparatus and method of network interface for data transmission. More particularly, this invention relates to an improved bridge-connector for an inter-network connection provided to dynamically prioritize data frame transfer such that transmission congestion in the network can be minimized.
2. Description of the Prior Art
Data transmission from a high speed network to a low speed network by the use of inter-network connecting devices, e.g., a bridge connector, often encounters the difficulties that transmission congestion may force a network time-out and cause even worse transmission congestion problems. This problem is caused by a conventional data transmission scheme which applies a first-in-first-out (FIFO) prioritization algorithm. This FIFO algorithm is most broadly employed in local area network (LAN), wide area network (WAN), or a connection between FDDI and Ethernet. In the network connections, the conventional FIFO transmission scheme transmits each of the output frames according to the sequence based on the order as these frames are received. When there are greater mismatches between the speeds of the high speed and low speed network, a frame congestion situation is often generated on the side of the low speed network. In the case when a buffer used to store the incoming frames temporarily is filled, the frames that arrive late have to be discarded. A time out for data transmission is often necessary when frames are discarded. Although the problem of a time out is resolved, more data are accumulated during the period when the network is out of service. Greater amount of data are to be transmitted as a result of the time out which often causes an even worse congestion.
FIGS. 1A and 1B are functional diagrams for illustrating the timing and sequencing of frame transmission over a bridge connector 10. In FIG. 1A, the solid black lines are used to represent transmission of data frames from host computer A in a high speed network 20 to host computer B in a low speed computer network 30. For the purpose of illustration, a line in the vertical direction represents the time sequence and the width in the horizontal direction represents bandwidth. A normal transmission of frames are shown FIG. 1A where three frames A1, A2, and A3 are transmitted from the high speed network 20 to the low speed network 30 via the bridge connector 10. The steeper slopes of A1, A2, and A3 on the right-hand side of the bridge connector 10 represent the fact that longer periods of time are required to transmit the frames on the low speed network 30. The periods T1 and T2 represent the time required to send a frame from host computer A to host computer B and to transmit acknowledge signal from computer B to computer A respectively. After the acknowledge signals are received back from the low speed network 30, more frames are transmitted from the high speed network 20 to the low speed network 30 as shown in FIG. 1A. This is a typical first-in-first-out (FIFO) transmission process carried out by the bridge connector 10.
A data transmission congestion is shown in FIG. 1B in which the high speed network 20 includes five host computers, computers A, B, C, D, and E, each transmitting data frames, e.g., A1, B1, C1, D1, E1, A2, A3, and A4, to the low speed network 30. A frame congestion is generated due to the transmission of five frames on the low speed network 30. Assume that a communication is carried out between host A in the HSN 20 and a host G in the LSN 30. Assume also that host-A and host-G use a sliding-windows based on a protocol with "go-back-N" type of error correction for data communication. With the window size of four, a maximum of three outstanding frames between host-A and host-G are allowed. As shown in FIG. 1B, the host-A is sending a series of frames A1, A2, A3, etc. to host-G. The first frame Al is sent without delay. Based on the first-in-first-out rule, transmission of A2, A3, and A4 has to wait for transmission of B1, C1, D1, and E1. As the host-G in the LSN sends the acknowledge signal back to HSN, the time allowed for sending A2 and A3 from host-A is over. The host-A then sends frame A4 instead. Re-transmission of frames A2 and A3 from host-A has to be performed later. In addition to the frames transmitted from host computer A, similar situation with frames generated by other computers B, C, D, and E may also occur thus causing further delay and congestion. Re-transmissions of many frames would be required which leads to further degradation of the network performance. When several hosts of the high speed network, e.g., hosts B, C, D, and E, are competing for a low speed data transmission channel, the FIFO priority management scheme applied by a conventional inter-network bridge for handling the timing and sequences of data transmission often causes uncertainties and delays due to the congestion problems. Wastes of precious bandwidth resources are caused by applying the first-in-first-out rule strictly and indiscretionarily.
Therefore, a need still exists in the art of computer network for data transmission to provide a new and improved method which can more effectively utilize the bandwidth of the low speed network to minimize data congestion thus preventing undesirable time-outs so that the difficulties encountered in the art can be resolved.